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TC7662B Datasheet PDF : 9 Pages
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CHARGE PUMP DC-TO-DC
VOLTAGE CONVERTER
TC7662B
4. When using polarized capacitors in the inverting mode,
the + terminal of C1 must be connected to pin 2 of the
TC7662B and the – terminal of C2 must be connected
to GND.
5. If the voltage supply driving the TC7662B has a large
source impedance (25-30 ohms), then a 2.2µF capaci-
tor from pin 8 to ground may be required to limit the
rate of rise of the input voltage to less than 2V/µsec.
TYPICAL APPLICATIONS
Simple Negative Voltage Converter
The majority of applications will undoubtedly utilize the
TC7662B for generation of negative supply voltages. Figure
3 shows typical connections to provide a negative supply
where a positive supply of +1.5V to +15V is available. Keep
in mind that pin 6 (LV) is tied to the supply negative (GND)
for supply voltages below 3.5 volts.
V+
10 µF
+
–
1
8
2
7
3 TC7662B 6
4
5
10 µF–
+
a.
VOUT = –V+
RO
–
V+
+
VOUT
b.
Figure 3. Simple Negative Converter and its Output Equivalent
The output characteristics of the circuit in Figure 3 can
be approximated by an ideal voltage source in series with a
resistance as shown in Figure 3b. The voltage source has a
value of–(V+). The output impedance (RO) is a function of
the ON resistance of the internal MOS switches (shown in
Figure 2), the switching frequency, the value of C1 and C2,
and the ESR (equivalent series resistance) of C1 and C2. A
good first order approximation for RO is:
RO ≅ 2(RSW1 + RSW3 + ESRC1) + 2(RSW2 + RSW4 +
ESRC1) +
1
+ ESRC2
fPUMP x C1
(fPUMP = fOSC , RSWX = MOSFET switch resistance)
2
Combining the four RSWX terms as RSW, we see that:
RO ≅ 2 x RSW +
1
+ 4 x ESRC1 + ESRC2Ω
fPUMP x C1
RSW, the total switch resistance, is a function of supply
4-86
voltage and temperature (See the Output Source Resis-
tance graphs), typically 23Ω at +25°C and 5V. Careful
selection of C1 and C2 will reduce the remaining terms,
minimizing the output impedance. High value capacitors will
reduce the 1/(fPUMP x C1) component, and low ESR capaci-
tors will lower the ESR term. Increasing the oscillator fre-
quency will reduce the 1/(fPUMP x C1) term, but may have the
side effect of a net increase in output impedance when C1 >
10µF and there is not enough time to fully charge the
capacitors every cycle. In a typical application when fOSC =
10kHz and C = C1 = C2 = 10µF:
RO ≅ 2 x 23 +
1
(5 x 103 x 10 x 10-6)
+ 4 x ESRC1 + ESRC2
RO ≅ (46 + 20 + 5 x ESRC) Ω
Since the ESRs of the capacitors are reflected in the
output impedance multiplied by a factor of 5, a high value
could potentially swamp out a low 1/(fPUMP x C1) term,
rendering an increase in switching frequency or filter capaci-
tance ineffective. Typical electrolytic capacitors may have
ESRs as high as 10Ω.
Output Ripple
ESR also affects the ripple voltage seen at the output.
The total ripple is determined by 2 voltages, A and B, as
shown in Figure 4. Segment A is the voltage drop across the
ESR of C2 at the instant it goes from being charged by C1
(current flowing into C2) to being discharged through the
load (current flowing out of C2). The magnitude of this
current change is 2 x IOUT, hence the total drop is 2 x IOUT x
ESRC2 volts. Segment B is the voltage change across C2
during time t2, the half of the cycle when C2 supplies current
to the load. The drop at B is IOUT x t2/C2 volts. The peak-to-
peak ripple voltage is the sum of these voltage drops:
( VRIPPLE ≅
1
) + ESRC2 x IOUT
2 x fPUMP x C2
t2
t1
0
V
– (V+)
B
A
Figure 4. Output Ripple
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